drm/i915: remove excess functions from plane protection check

Reduce the function calls by reusing ->decrypt.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231109160722.3372379-2-jani.nikula@intel.com